The present invention relates to a projection lens. More particularly, the invention relates to a projection lens which has a mirror (reflecting mirror) type resinous curved surface and is for use in high-precision high-resolution projection devices, especially rear-projection television receivers, and the like. This projection lens is applicable also to projection devices such as overhead projectors, window displays, and front-data projectors.
By far the most common materials for use as large optical parts required to have high precision, such as the projection lens mentioned above, have been glasses and metallic materials such as aluminum and steels. This is because optical parts having excellent optical precision are obtainable from such materials, in particular, images reduced in distortion are obtained even when temperature changes, and because such materials have high suitability for precision cutting/polishing.
However, use of glasses or metallic materials such as aluminum and steels as optical parts as described above poses the following problems.
First, in the case of using glass materials, expensive production equipment is necessary. Namely, since a glass is press-molded with heating, it is necessary to employ an apparatus having the function of evenly heating the glass to about 700xc2x0 C. or higher for sufficiently melting the glass and to use a mold having high shape precision even under high-temperature compression and further having durability. Furthermore, a prolonged cycle time is necessary when heating/cooling time is taken into account, and this results in low productivity and an exceedingly high product cost. Because of these, glass parts for projection devices are usable only in highly special applications where high costs are acceptable, and have not spread to general office use or domestic use.
Next, in the case of using metallic materials such as aluminum and steels, it is basically necessary to produce optical parts through precise cutting and polishing for each part. Because of this, the parts themselves are considerably expensive in this case also and unsuitable for mass production.
It is hence expected to use resin materials which are relatively easy to mold, in place of glasses or metallic materials such as aluminum and steels. However, ordinary injection-molded products or compression-molded products formed from thermoplastic resins suffer considerable molding shrinkage and have warpage, sink marks, or failures concerning shape (curved surface) transfer from the mold. Because of this, it is difficult to obtain a molded product, such as a projection lens, which has a large size (large area and large thickness) and high shape precision so as to obtain high-resolution image quality.
On the other hand, in the case where a resin curing with heat or light, such as an epoxy or diacrylate resin, is used as a feed material, cure shrinkage occurs to pose a problem that the molded product suffers deformation, distortion, and the like due to the shrinkage as in the case of thermoplastic resins. In addition, use of such curable resins has a problem that since it necessitates a prolonged time period of from several hours to tens of hours for curing or for annealing (curing acceleration, crosslink density improvement, and relaxation of residual stress) after initial cure, it is unsuitable for mass production.
The present inventors made intensive investigations in view of the problems described above. As a result, they have found that even when a resin is used as a feed material, a resin base having a high-precision reflecting surface reduced in distortion can be obtained by directing attention to a specific property and selecting a resin material so that the value of this property is within a specific range, and by controlling molding conditions. They have further found that a high-performance projection lens can be obtained by using the resin base. The invention has been thus completed. Namely, an essential aspect of the invention relates to a projection lens comprising a resin base having a given curved surface and a reflecting layer formed over the surface of the resin base, characterized in that the average of in-plane birefringent phase differences per unit thickness as measured with incident light from the direction perpendicular to the curved optical functional surface of the resin base is 30 nm/mm or less in a region accounting for at least 60% of the area of the optical functional surface.